Why is my Apple Watch recording below average cardio fitness after a software update?

My watch has started recording my cardio fitness again but it is showing me as below average when I’ve always been above average. I’m not doing anything radically different so what may be causing this result? Is it a watch software problem?

Ever since I bought the watch, series 8, I have always had an above average cardio fitness average. Last month my cardio crashed 10 points and then stopped recording for a week, it has now restarted but still recording below average readings. Is this a software issue, I have just updated my software to 11.3.1 and it hasn’t fixed my problem. Is there a workaround for this problem? Thankyou

[Re-Titled by Moderator]